This invention relates generally to improvements in methods and apparatus for forming three-dimensional objects from a fluid medium and, more particularly, to new and improved stereolithography systems involving the application of enhanced data manipulation and lithographic techniques to production of three-dimensional objects, whereby such objects can be formed more rapidly, reliably, accurately and economically.
It is common practice in the production of plastic parts and the like to first design such a part and then painstakingly produce a prototype of the part, all involving considerable time, effort and expense. The design is then reviewed and, oftentimes, the laborious process is again and again repeated until the design has been optimized. After design optimization, the next step is production. Most production plastic parts are injection molded. Since the design time and tooling costs are very high, plastic parts are usually only practical in high volume production. While other processes are available for the production of plastic parts, including direct machine work, vacuum-forming and direct forming, such methods are typically only cost effective for short run production, and the parts produced are usually inferior in quality to molded parts.
Very sophisticated techniques have been developed in the past for generating three-dimensional objects within a fluid medium which is selectively cured by beams of radiation brought to selective focus at prescribed intersection points within the three-dimensional volume of the fluid medium. Typical of such three-dimensional systems are those described in U.S. Pat. Nos. 4,041,476; 4,078,229; 4,238,840 and 4,288,861. All of these systems rely upon the buildup of synergistic energization at selected points deep within the fluid volume, to the exclusion of all other points in the fluid volume. Unfortunately, however, such three-dimensional forming systems face a number of problems with regard to resolution and exposure control. The loss of radiation intensity and image forming resolution of the focused spots as the intersections move deeper into the fluid medium create rather obvious complex control situations. Absorption, diffusion, dispersion and diffraction all contribute to the difficulties of working deep within the fluid medium on an economical and reliable basis.
In recent years, "stereolithography" systems, such as those described in U.S. Pat. No. 4,575,330 entitled "Apparatus For Production Of Three-Dimensional Objects By Stereolithography" have come into use. Basically, stereolithography is a method for automatically building complex plastic parts by successively printing cross-sections of photopolymer (such as liquid plastic) on top of each other until all of the thin layers are joined together to form a whole part. With this technology, the parts are literally grown in a vat of liquid plastic. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form and for making prototypes.
Photocurable polymers change from liquid to solid in the presence of light and their photospeed with ultraviolet light (ULV) is fast enough to make them practical model building materials. The material that is not polymerized when a part is made is still usable and remains in the vat as successive parts are made. An ultraviolet laser generates a small intense spot of UV. This spot is moved across the liquid surface with a galvanometer mirror X-Y scanner. The scanner is driven by computer generated vectors or the like. Precise complex patterns can be rapidly produced with this technique.
The laser scanner, the photopolymer vat and the elevator, along with a controlling computer, combine together to form a stereolithography apparatus, referred to as "SLA". An SLA is programmed to automatically make a plastic part by drawing are cross section at a time, and building it up layer by layer.
Stereolithography represents an unprecedented way to quickly make complex or simple parts without tooling. Since this technology depends on using a computer to generate its cross sectional patterns, there is a natural data link to CAD/CAM. However, such systems have encountered difficulties relating to shrinkage, curl and other distortions, as well as resolution, accuracy and difficulties in producing certain object shapes.
Objects built using stereolithography have a tendency to distort from their CAD designed dimensions. This distortion may or may not appear in a specific object, based on how much stress is developed by the specific cure parameters and on the object's ability to withstand stress. The stress that causes distortion develops when material that is being converted from liquid to solid comes into contact with and bonds to previously cured material. When material is converted from liquid to solid it shrinks slightly. This shrinking causes stress and has two primary physical causes: (1) density of the liquid is less than that of the solid plastic; and (2) the chemical reaction that causes the change of state is strongly exothermic causing the curing material to thermally expand and contract.
Certain sections of an object will be able to resist stresses without any apparent warp (stress is at a tolerable level). On the other hand, other sections may distort considerably as the stress and structural strength balance each other. Since stress is caused by contact between curing material and cured material it can be propagated along the entire length of contact between the curing line and cured material. Most contact of curing to cured material occurs from one layer to the next as opposed to along a single layer. This implies most distortions will be vertical in nature as opposed to horizontal. Therefore, there has been a need for a technique to reduce vertical distortions.
"Birdnesting" is a phenomena that can occur on parts that require down-facing, near-flat skin by the stereolithographic's slicing program. Areas require down-facing, near-flat skin because their boundary vectors do not have any support when they are drawn. By the time cross-hatch is finally drawn, to secure the boundaries, the boundary vectors may have moved away from their proper positions and, therefore, may not be secured at particular locations. These unsecured boundaries can move up and down and give a rough surface finish to the object, similar to a bird's nest.
Hence, workers in the art have recognized the need for a solution to the aforedescribed problems encountered in stereolithographics, and there continues to be a long existing need in the design and production arts for the capability of rapidly and reliably moving from the design stage to the prototype stage and to ultimate production, particularly moving directly from the computer designs for such plastic parts to virtually immediate prototypes and the facility for large scale production on an economical and automatic basis.
Accordingly, those concerned with the development and production of three-dimensional plastic objects and the like have long recognized the desirability for further improvement in more rapid, reliable, economical and automatic means which would facilitate quickly moving from a design stage to the prototype stage and to production, while avoiding the problems of stress, distortion and poor part finish. The present invention clearly fullfills all of these needs.